Wireless standard 802.11 b. All existing Wi-Fi network standards

The IEEE (Institute of Electrical and Electronic Engineers) is developing WiFi 802.11 standards.

IEEE 802.11 is the base standard for Wi-Fi networks, which defines a set of protocols for the lowest transfer rates.

IEEE 802.11b- describes b O higher transmission speeds and introduces more technological restrictions. This standard was widely promoted by WECA ( Wireless Ethernet Compatibility Alliance ) and was originally called WiFi .
Frequency channels in the 2.4GHz spectrum are used ().
Ratified in 1999.
RF technology used: DSSS.
Coding: Barker 11 and CCK.
Modulations: DBPSK and DQPSK,
Maximum data transfer rates (transfer) in the channel: 1, 2, 5.5, 11 Mbps,

IEEE 802.11a- describes significantly higher transfer rates than 802.11b.
Frequency channels in the 5GHz frequency spectrum are used. ProtocolNot compatible with 802.11 b.
Ratified in 1999.
RF technology used: OFDM.
Coding: Conversion Coding.
Modulations: BPSK, QPSK, 16-QAM, 64-QAM.
Maximum data transfer rates in the channel: 6, 9, 12, 18, 24, 36, 48, 54 Mbps.

IEEE 802.11g - describes data transfer rates equivalent to 802.11a.
Frequency channels in the 2.4GHz spectrum are used. The protocol is compatible with 802.11b.
Ratified in 2003.
RF technologies used: DSSS and OFDM.
Coding: Barker 11 and CCK.
Modulations: DBPSK and DQPSK,
Maximum data transfer rates (transfer) in the channel:
- 1, 2, 5.5, 11 Mbps on DSSS and
- 6, 9, 12, 18, 24, 36, 48, 54 Mbps on OFDM.

IEEE 802.11n- the most advanced commercial WiFi standard, on this moment, officially approved for import and use on the territory of the Russian Federation (802.11ac is still under development by the regulator). 802.11n uses frequency channels in the 2.4GHz and 5GHz WiFi frequency spectrums. Compatible with 11b/11 a/11g . Although it is recommended to build networks targeting only 802.11n, because... requires configuration of special protective modes if backward compatibility with legacy standards is required. This leads to a large increase in signal information anda significant reduction in the available useful performance of the air interface. Actually, even one WiFi client 802.11g or 802.11b will require special settings the entire network and its immediate significant degradation in terms of aggregated performance.
The WiFi 802.11n standard itself was released on September 11, 2009.
WiFi frequency channels with a width of 20MHz and 40MHz (2x20MHz) are supported.
RF technology used: OFDM.
OFDM MIMO (Multiple Input Multiple Output) technology is used up to the 4x4 level (4xTransmitter and 4xReceiver). In this case, a minimum of 2xTransmitter per Access Point and 1xTransmitter per user device.
Examples of possible MCS (Modulation & Coding Scheme) for 802.11n, as well as the maximum theoretical transfer rates in the radio channel are presented in the following table:

Here SGI is the guard intervals between frames.
Spatial Streams is the number of spatial streams.
Type is the modulation type.
Data Rate is the maximum theoretical data transfer rate in the radio channel in Mbit/sec.

It is important to emphasize that the indicated speeds correspond to the concept of channel rate and are the limiting value using this set technologies within the framework of the described standard (in fact, these values, as you probably noticed, are written by manufacturers on the boxes of home WiFi devices in stores). But in real life these values are not achievable due to the specifics of the WiFi 802.11 standard technology itself. For example, “political correctness” is strongly influenced here in terms of ensuring CSMA/CA ( WiFi devices constantly listen to the air and cannot transmit if the transmission medium is busy), the need to acknowledge each unicast frame, the half-duplex nature of all WiFi standards and only 802.11ac/Wave-2 can begin to bypass this, etc. Therefore, the practical effectiveness of outdated 802.11 standards b/g/a never exceeds 50% in ideal conditions (for example for 802.11g maximum speed per subscriber is usually no higher than 22Mb/s), and for 802.11n the efficiency can be up to 60%. If the network operates in protected mode, which often happens due to the mixed presence of different WiFi chips on various devices ah in the network, then even the indicated relative efficiency can drop by 2-3 times. This applies, for example, to a mix of Wi-Fi devices with 802.11b, 802.11g chips on a network with 802.11g WiFi access points or 802.11g/802.11b WiFi devices on a network with 802.11n WiFi access points, etc. More information about .

In addition to the basic WiFi standards 802.11a, b, g, n, additional standards exist and are used to implement various service functions:

. 802.11d. To adapt various WiFi standard devices to specific country conditions. Within the regulatory framework of each state, ranges often vary and may even differ depending on geographic location. IEEE 802.11d WiFi standard allows adjustment of frequency bands in devices different manufacturers using special options introduced into the media access control protocols.

. 802.11e. Describes QoS quality classes for the transmission of various media files and, in general, various media content. Adaptation of the MAC layer for 802.11e determines the quality, for example, of simultaneous transmission of audio and video.

. 802.11f. Aimed at unifying the parameters of Wi-Fi access points various manufacturers. The standard allows the user to work with different networks when moving between coverage areas of individual networks.

. 802.11h. Used to prevent problems with weather and military radars by dynamically reducing the emitted power of Wi-Fi equipment or dynamically switching to another frequency channel when a trigger signal is detected (in most European countries, ground stations tracking weather and communications satellites, as well as military radars operate in ranges close to 5 MHz). This standard is a necessary ETSI requirement for equipment approved for use in the European Union.

. 802.11i. The first iterations of the 802.11 WiFi standards used the WEP algorithm to secure Wi-Fi networks. It was believed that this method could provide confidentiality and protection of the transmitted data of authorized wireless users from eavesdropping. Now this protection can be hacked in just a few minutes. Therefore, the 802.11i standard developed new methods for protecting Wi-Fi networks, implemented at both the physical and software levels. Currently, to organize a security system in Wi-Fi 802.11 networks, it is recommended to use Wi-Fi Protected Access (WPA) algorithms. They also provide compatibility between wireless devices various standards and various modifications. WPA protocols use an advanced RC4 encryption scheme and a mandatory authentication method using EAP. The stability and security of modern Wi-Fi networks is determined by privacy verification and data encryption protocols (RSNA, TKIP, CCMP, AES). The most recommended approach is to use WPA2 with AES encryption (and don't forget about 802.1x using tunneling mechanisms, such as EAP-TLS, TTLS, etc.). .

. 802.11k. This standard is actually aimed at implementing load balancing in the radio subsystem Wi-Fi networks. Typically, in a wireless LAN, the subscriber device usually connects to the access point that provides the strongest signal. This often leads to network congestion at one point, when many users connect to one Access Point at once. To control such situations, the 802.11k standard proposes a mechanism that limits the number of subscribers connected to one Access Point and makes it possible to create conditions under which new users will join another AP even despite a weaker signal from it. In this case, the aggregated throughput network increases due to more efficient use of resources.

. 802.11m. Amendments and corrections for the entire group of 802.11 standards are combined and summarized in a separate document under the general name 802.11m. The first release of 802.11m was in 2007, then in 2011, etc.

. 802.11p. Determines the interaction of Wi-Fi equipment moving at speeds of up to 200 km/h past stationary WiFi Access Points located at a distance of up to 1 km. Part of the Wireless Access in Vehicular Environment (WAVE) standard. WAVE standards define an architecture and a complementary set of utility functions and interfaces that provide a secure radio communications mechanism between moving vehicles. These standards are developed for applications such as traffic management, traffic safety monitoring, automated payment collection, vehicle navigation and routing, etc.

. 802.11s. A standard for implementing mesh networks (), where any device can serve as both a router and an access point. If the nearest access point is overloaded, data is redirected to the nearest unloaded node. In this case, a data packet is transferred (packet transfer) from one node to another until it reaches its final destination. This standard introduces new protocols at the MAC and PHY levels that support broadcast and multicast transmission (transfer), as well as unicast delivery over a self-configuring point system Wi-Fi access. For this purpose, the standard introduced a four-address frame format. Implementation examples WiFi networks Mesh: , .

. 802.11t. The standard was created to institutionalize the process of testing solutions of the IEEE 802.11 standard. Testing methods, methods of measurement and processing of results (treatment), requirements for testing equipment are described.

. 802.11u. Defines procedures for interaction of Wi-Fi standard networks with external networks. The standard must define access protocols, priority protocols and prohibition protocols for working with external networks. At the moment, a large movement has formed around this standard, both in terms of developing solutions - Hotspot 2.0, and in terms of organizing inter-network roaming - a group of interested operators has been created and is growing, who jointly resolve roaming issues for their Wi-Fi networks in dialogue (WBA Alliance ). Read more about Hotspot 2.0 in our articles: , .

. 802.11v. The standard should include amendments aimed at improving the network management systems of the IEEE 802.11 standard. Modernization at the MAC and PHY levels should allow the configuration of client devices connected to the network to be centralized and streamlined.

. 802.11y. Additional communication standard for the frequency range 3.65-3.70 GHz. Designed for latest generation devices operating with external antennas at speeds up to 54 Mbit/s at a distance of up to 5 km in open space. The standard is not fully completed.

802.11w. Defines methods and procedures for improving the protection and security of the media access control (MAC) layer. The standard protocols structure a system for monitoring data integrity, the authenticity of their source, the prohibition of unauthorized reproduction and copying, data confidentiality and other protection measures. The standard introduces management frame protection (MFP: Management Frame Protection), and additional security measures help neutralize external attacks, such as DoS. A little more on MFP here: . In addition, these measures will ensure security for the most sensitive network information that will be transmitted over networks that support IEEE 802.11r, k, y.

802.11ac. A new WiFi standard that operates only in the 5GHz frequency band and provides significantly faster O higher speeds as an individual WiFi client, and to the WiFi Access Point. See our article for more details.

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The new IEEE 802.11n wireless standard has been talked about for several years now. This is understandable, because one of the main disadvantages of the existing IEEE 802.11a/b/g wireless communication standards is the data transfer speed is too low. Indeed, the theoretical throughput of the IEEE 802.11a/g protocols is only 54 Mbit/s, and the actual data transfer rate does not exceed 25 Mbit/s. The new wireless communication standard IEEE 802.11n should provide transmission speeds of up to 300 Mbit/s, which looks very tempting compared to 54 Mbit/s. Of course, the actual data transfer rate in the IEEE 802.11n standard, as test results show, does not exceed 100 Mbit/s, but even in this case, the actual data transfer speed is four times higher than in the IEEE 802.11g standard. The IEEE 802.11n standard has not yet been fully adopted (this should happen before the end of 2007), but almost all wireless equipment manufacturers have already begun producing devices compatible with the Draft version of the IEEE 802.11n standard.
In this article we will look at the basic provisions of the new IEEE 802.11n standard and its main differences from the 802.11a/b/g standards.

We have already talked about the 802.11a/b/g wireless communication standards in some detail on the pages of our magazine. Therefore, in this article we will not describe them in detail; however, in order for the main differences between the new standard and its predecessors to be obvious, we will have to make a digest of previously published articles on this topic.

Looking at the history of wireless standards used to create wireless local networks(Wireless Local Area Network, WLAN), it is probably worth remembering the IEEE 802.11 standard, which, although no longer found in its pure form, is the progenitor of all other wireless communication standards for WLAN networks.

IEEE 802.11 standard

The 802.11 standard provides for the use of a frequency range from 2400 to 2483.5 MHz, that is, an 83.5 MHz wide range divided into several frequency subchannels.

The 802.11 standard is based on the technology of spreading the spectrum (Spread Spectrum, SS), which implies that the initially narrow-band (in terms of spectrum width) useful information signal is converted during transmission in such a way that its spectrum is much wider than the spectrum of the original signal. Simultaneously with the broadening of the signal spectrum, a redistribution of the spectral energy density of the signal occurs - the signal energy is also “spread out” across the spectrum.

The 802.11 protocol uses Direct Sequence Spread Spectrum (DSSS) technology. Its essence lies in the fact that to broaden the spectrum of an initially narrow-band signal, a chip sequence, which is a sequence of rectangular pulses, is built into each transmitted information bit. If the duration of one chip pulse is n times less than the duration of the information bit, then the width of the spectrum of the converted signal will be n times the width of the spectrum of the original signal. In this case, the amplitude of the transmitted signal will decrease by n once.

The chip sequences embedded in the information bits are called noise-like codes (PN-sequences), which emphasizes the fact that the resulting signal becomes noise-like and is difficult to distinguish from natural noise.

It’s clear how to broaden the signal spectrum and make it indistinguishable from natural noise. To do this, in principle, you can use an arbitrary (random) chip sequence. However, the question arises of how to receive such a signal. After all, if it becomes noise-like, then isolating a useful information signal from it is not so easy, if not impossible. Nevertheless, this can be done, but for this you need to select the chip sequence accordingly. Chip sequences used to broaden the signal spectrum must satisfy certain autocorrelation requirements. In mathematics, autocorrelation refers to the degree to which a function is similar to itself at different points in time. If you select a chip sequence for which the autocorrelation function will have a pronounced peak for only one point in time, then such an information signal can be distinguished at the noise level. To do this, the received signal is multiplied by the chip sequence in the receiver, that is, the autocorrelation function of the signal is calculated. As a result, the signal again becomes narrow-band, so it is filtered in a narrow frequency band equal to twice the transmission rate. Any interference that falls within the band of the original broadband signal, after multiplication by the chip sequence, on the contrary, becomes broadband and is cut off by filters, and only part of the interference falls into the narrow information band; its power is significantly less than the interference acting at the receiver input.

There are quite a lot of chip sequences that meet the specified autocorrelation requirements, but the so-called Barker codes are of particular interest to us, since they are used in the 802.11 protocol. Barker codes have the best known codes pseudorandom sequences noise-like properties, which led to their widespread use. The 802.11 family of protocols uses Barker code that is 11 chips long.

In order to transmit a signal, the information sequence of bits in the receiver is added modulo 2 (mod 2) with the 11-chip Barker code using an XOR (exclusive OR) gate. Thus, a logical one is transmitted by a direct Barker sequence, and a logical zero by an inverse sequence.

The 802.11 standard provides two speed modes - 1 and 2 Mbit/s.

With an information speed of 1 Mbit/s, the speed of individual Barker sequence chips is 11x106 chips per second, and the spectrum width of such a signal is 22 MHz.

Considering that the width of the frequency range is 83.5 MHz, we find that a total of three non-overlapping frequency channels can fit in this frequency range. The entire frequency range, however, is usually divided into 11 overlapping frequency channels of 22 MHz, spaced 5 MHz from each other. For example, the first channel occupies the frequency range from 2400 to 2423 MHz and is centered relative to the frequency of 2412 MHz. The second channel is centered relative to the frequency of 2417 MHz, and the last, 11th channel is centered relative to the frequency of 2462 MHz. When viewed this way, channels 1, 6 and 11 do not overlap with each other and have a 3 MHz gap relative to each other. It is these three channels that can be used independently of each other.

To modulate a sinusoidal carrier signal at a data rate of 1 Mbit/s, relative binary phase modulation (DBPSK) is used.

In this case, information encoding occurs due to a phase shift of the sinusoidal signal relative to the previous signal state. Binary phase modulation provides two possible phase shift values - 0 and p. Then a logical zero can be transmitted by an in-phase signal (the phase shift is 0), and a logical one can be transmitted by a signal that is phase shifted by p.

An information speed of 1 Mbit/s is mandatory in the IEEE 802.11 standard (Basic Access Rate), but a speed of 2 Mbit/s (Enhanced Access Rate) is optionally possible. To transmit data at this speed, the same DSSS technology with 11-chip Barker codes is used, but Differential Quadrature Phase Shift Key is used to modulate the carrier wave.

In conclusion, considering the physical layer of the 802.11 protocol, we note that at an information speed of 2 Mbit/s, the speed of individual chips of the Barker sequence remains the same, that is, 11x106 chips per second, and therefore the width of the spectrum of the transmitted signal does not change.

IEEE 802.11b standard

The IEEE 802.11 standard was replaced by the IEEE 802.11b standard, which was adopted in July 1999. This standard is a kind of extension of the basic 802.11 protocol and, in addition to speeds of 1 and 2 Mbit/s, provides speeds of 5.5 and 11 Mbit/s, for which so-called complementary codes (Complementary Code Keying, CCK) are used.

Complementary codes, or CCK sequences, have the property that the sum of their autocorrelation functions for any cyclic shift other than zero is always zero, so they, like Barker codes, can be used to recognize a signal from a background of noise.

The main difference between CCK sequences and the previously discussed Barker codes is that there is not a strictly defined sequence through which either a logical zero or a one can be encoded, but a whole set of sequences. This circumstance makes it possible to encode several information bits in one transmitted symbol and thereby increases the information transmission speed.

The IEEE 802.11b standard deals with complex complementary 8-chip sequences defined on a set of complex elements taking values (1, –1, +j, –j}.

Complex signal representation is a convenient mathematical tool for representing a phase-modulated signal. Thus, a sequence value equal to 1 corresponds to a signal in phase with the generator signal, and a sequence value equal to –1 corresponds to an antiphase signal; sequence value equal j- a signal phase-shifted by p/2, and the value is equal to – j, - signal phase shifted by –p/2.

Each element of the CCK sequence is a complex number, the value of which is determined using a rather complex algorithm. There are a total of 64 sets of possible CCK sequences, with the choice of each determined by the sequence of input bits. To uniquely select one CCK sequence, six input bits are required. Thus, the IEEE 802.11b protocol uses one of 64 possible eight-bit CKK sequences when encoding each character.

At a speed of 5.5 Mbit/s, 4 bits of data are simultaneously encoded in one symbol, and at a speed of 11 Mbit/s - 8 bits of data. In both cases, the symbolic transmission rate is 1.385x106 symbols per second (11/8 = 5.5/4 = 1.385), and taking into account that each character is specified by an 8-chip sequence, we find that in both cases the transmission speed of individual chips is 11x106 chips per second. Accordingly, the signal spectrum width at speeds of both 11 and 5.5 Mbit/s is 22 MHz.

IEEE 802.11g standard

Two competing technologies were considered during the development of the 802.11g standard: the orthogonal frequency division OFDM method, borrowed from the 802.11a standard and proposed by Intersil, and the binary packet convolutional coding method PBCC, proposed by Texas Instruments. As a result, the 802.11g standard contains a compromise solution: OFDM and CCK technologies are used as base technologies, and the optional use of PBCC technology is provided.

The idea of convolutional coding (Packet Binary Convolutional Coding, PBCC) is as follows. The incoming sequence of information bits is converted in a convolutional encoder so that each input bit corresponds to more than one output bit. That is, the convolutional encoder adds certain redundant information to the original sequence. If, for example, each input bit corresponds to two output bits, then we talk about convolutional coding with a speed r= 1/2. If every two input bits correspond to three output bits, then it will be 2/3.

Any convolutional encoder is built on the basis of several sequentially connected memory cells and XOR gates. The number of storage cells determines the number of possible encoder states. If, for example, a convolutional encoder uses six memory cells, then the encoder stores information about six previous states signal, and taking into account the value of the incoming bit, we find that such an encoder uses seven bits of the input sequence. Such a convolutional encoder is called a seven-state encoder ( K = 7).

The output bits generated by the convolutional encoder are determined by XOR operations between the values of the input bit and the bits stored in the storage cells, that is, the value of each output bit generated depends not only on the incoming information bit, but also on several previous bits.

PBCC technology uses seven-state convolutional encoders ( K= 7) with speed r = 1/2.

The main advantage of convolutional encoders is the noise immunity of the sequence they generate. The fact is that with redundant coding, even in the event of reception errors, the original bit sequence can be accurately restored. A Viterbi decoder is used at the receiver side to restore the original bit sequence.

The dibit generated in the convolutional encoder is subsequently used as a transmitted symbol, but it is first subjected to phase modulation. Moreover, depending on the transmission speed, binary, quadrature or even eight-position phase modulation is possible.

Unlike DSSS technologies (Barker codes, SSK sequences), convolutional coding technology does not use spectrum broadening technology through the use of noise-like sequences, however, spectrum broadening to standard 22 MHz is also provided in this case. To do this, variations of possible QPSK and BPSK signal constellations are used.

The considered PBCC coding method is optionally used in the 802.11b protocol at speeds of 5.5 and 11 Mbit/s. Similarly, in the 802.11g protocol for transmission speeds of 5.5 and 11 Mbit/s, this method is also used optionally. In general, due to the compatibility of the 802.11b and 802.11g protocols, the encoding technologies and speeds provided by the 802.11b protocol are also supported in the 802.11g protocol. In this regard, up to a speed of 11 Mbps, the 802.11b and 802.11g protocols are the same, except that the 802.11g protocol provides speeds that the 802.11b protocol does not.

Optionally, in the 802.11g protocol, PBCC technology can be used at transmission rates of 22 and 33 Mbit/s.

For a speed of 22 Mbit/s, compared to the PBCC scheme we have already considered, data transmission has two features. First of all, 8-position phase modulation (8-PSK) is used, that is, the phase of the signal can take on eight different values, which allows three bits to be encoded in one symbol. In addition, a puncture encoder (Puncture) has been added to the circuit, with the exception of the convolutional encoder. The meaning of this solution is quite simple: the redundancy of the convolutional encoder, equal to 2 (for each input bit there are two output bits), is quite high and under certain noise conditions it is unnecessary, so the redundancy can be reduced so that, for example, every two input bits correspond to three output bits . For this, you can, of course, develop an appropriate convolutional encoder, but it is better to add a special puncture encoder to the circuit, which will simply destroy extra bits.

Let's say a puncture encoder removes one bit from every four input bits. Then every four incoming bits will correspond to three outgoing ones. The speed of such an encoder is 4/3. If such an encoder is used in conjunction with a convolutional encoder with a rate of 1/2, then overall speed coding will already be 2/3, that is, every two input bits will correspond to three output bits.

As already noted, PBCC technology is optional in the IEEE 802.11g standard, and OFDM technology is mandatory. In order to understand the essence of OFDM technology, let's take a closer look at the multipath interference that occurs when signals propagate in an open environment.

The effect of multipath signal interference is that, as a result of multiple reflections from natural obstacles, the same signal can reach the receiver in different ways. But different propagation paths differ from each other in length, and therefore the signal attenuation will not be the same for them. Consequently, at the receiving point, the resulting signal represents the interference of many signals having different amplitudes and shifted relative to each other in time, which is equivalent to the addition of signals with different phases.

The consequence of multipath interference is distortion of the received signal. Multipath interference is inherent in any type of signal, but it has a particularly negative effect on wideband signals, since when using a broadband signal, as a result of interference, certain frequencies add up in phase, which leads to an increase in the signal, and some, on the contrary, out of phase, causing a weakening of the signal at a given frequency.

Speaking about multipath interference that occurs during signal transmission, two extreme cases are noted. In the first of them, the maximum delay between signals does not exceed the duration of one symbol and interference occurs within one transmitted symbol. In the second, the maximum delay between signals is greater than the duration of one symbol, so as a result of interference, signals representing different symbols are added, and so-called inter-symbol interference (ISI) occurs.

It is intersymbol interference that has the most negative effect on signal distortion. Since a symbol is a discrete signal state characterized by the values of carrier frequency, amplitude and phase, the amplitude and phase of the signal change for different symbols, and therefore it is extremely difficult to restore the original signal.

For this reason, when high speeds transmission uses a data encoding method called Orthogonal Frequency Division Multiplexing (OFDM). Its essence lies in the fact that the stream of transmitted data is distributed over many frequency subchannels and transmission is carried out in parallel on all such subchannels. In this case, a high transmission speed is achieved precisely due to the simultaneous transmission of data over all channels, while the transmission speed in a separate subchannel may be low.

Due to the fact that the data transmission rate in each of the frequency subchannels can be made not too high, the prerequisites are created for effective suppression of intersymbol interference.

Frequency division of channels requires that an individual channel be narrow enough to minimize signal distortion, but at the same time wide enough to provide the required transmission speed. In addition, to economically use the entire bandwidth of a channel divided into subchannels, it is desirable to arrange the frequency subchannels as close to each other as possible, but at the same time avoid interchannel interference to ensure their complete independence. Frequency channels that satisfy the above requirements are called orthogonal. The carrier signals of all frequency subchannels are orthogonal to each other. It is important that the orthogonality of the carrier signals guarantees the frequency independence of the channels from each other, and therefore the absence of inter-channel interference.

This method of dividing a wideband channel into orthogonal frequency subchannels is called orthogonal frequency division multiplexing (OFDM). To implement it in transmitting devices, an inverse fast Fourier transform (IFFT) is used, which transforms the previously multiplexed n-channels signal from time O th representation into frequency.

One of the key advantages of the OFDM method is the combination of high transmission speed with effective resistance to multipath propagation. Of course, OFDM technology itself does not eliminate multipath propagation, but it creates the prerequisites for eliminating the effect of intersymbol interference. The fact is that an integral part of OFDM technology is the Guard Interval (GI) - a cyclic repetition of the end of the symbol, attached at the beginning of the symbol.

The guard interval creates pauses between individual symbols, and if its duration exceeds the maximum signal delay time due to multipath propagation, then intersymbol interference does not occur.

When using OFDM technology, the duration of the guard interval is one-fourth of the duration of the symbol itself. In this case, the symbol has a duration of 3.2 μs, and the guard interval is 0.8 μs. Thus, the duration of the symbol together with the guard interval is 4 μs.

Speaking about the OFDM frequency division technology used at various speeds in the 802.11g protocol, we have not yet touched upon the issue of the carrier signal modulation method.

The 802.11g protocol uses binary and quadrature phase modulation BPSK and QPSK at low bit rates. When using BPSK modulation, only one information bit is encoded in one symbol, and when using QPSK modulation, two information bits are encoded. BPSK modulation is used to transmit data at speeds of 6 and 9 Mbit/s, and QPSK modulation is used at speeds of 12 and 18 Mbit/s.

For transmission at higher speeds, quadrature amplitude modulation QAM (Quadrature Amplitude Modulation) is used, in which information is encoded by changing the phase and amplitude of the signal. The 802.11g protocol uses 16-QAM and 64-QAM modulation. The first modulation involves 16 different signal states, which allows 4 bits to be encoded in one symbol; the second - 64 possible signal states, which makes it possible to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at 24 and 36 Mbps, and 64-QAM modulation is used at 48 and 54 Mbps.

In addition to the use of CCK, OFDM and PBCC coding, the IEEE 802.11g standard also optionally provides various hybrid coding options.

In order to understand the essence of this term, remember that any transmitted data packet contains a header (preamble) with service information and a data field. When referring to a packet in CCK format, it means that the header and data of the frame are transmitted in CCK format. Similarly, with OFDM technology, the frame header and data are transmitted using OFDM encoding. Hybrid coding means that different coding technologies can be used for the frame header and data fields. For example, when using CCK-OFDM technology, the frame header is encoded using CCK codes, but the frame data itself is transmitted using multi-frequency OFDM encoding. Thus, CCK-OFDM technology is a kind of hybrid of CCK and OFDM. However, this is not the only hybrid technology - when using PBCC packet coding, the frame header is transmitted using CCK codes, and the frame data is encoded using PBCC.

IEEE 802.11a standard

The IEEE 802.11b and IEEE 802.11g standards discussed above refer to the 2.4 GHz frequency range (from 2.4 to 2.4835 GHz), and the IEEE 802.11a standard, adopted in 1999, involves the use of a higher frequency range (from 5 .15 to 5.350 GHz and 5.725 to 5.825 GHz). In the USA, this range is called the Unlicensed National Information Infrastructure (UNII) range.

In accordance with FCC rules, the UNII frequency range is divided into three 100-MHz sub-bands, differing in maximum emission power limits. The low band (5.15 to 5.25 GHz) provides only 50 mW of power, the middle (5.25 to 5.35 GHz) 250 mW, and the high (5.725 to 5.825 GHz) 1 W. The use of three frequency subbands with a total width of 300 MHz makes the IEEE 802.11a standard the most broadband of the 802.11 family of standards and allows the entire frequency range to be divided into 12 channels, each of which has a width of 20 MHz, with eight of them lying in the 200 MHz range from 5 .15 to 5.35 GHz, and the remaining four channels are in the 100 MHz range from 5.725 to 5.825 GHz (Fig. 1). At the same time, the four upper frequency channels, which provide the highest transmission power, are used primarily for transmitting signals outdoors.

Rice. 1. Division of the UNII range into 12 frequency subbands

The IEEE 802.11a standard is based on the Orthogonal Frequency Division Multiplexing (OFDM) technique. To separate the channels, an inverse Fourier transform is used with a window of 64 frequency subchannels. Since each of the 12 channels defined in the 802.11a standard is 20 MHz wide, each orthogonal frequency subchannel (subcarrier) is 312.5 kHz wide. However, out of 64 orthogonal subchannels, only 52 are used, with 48 of them used for data transmission (Data Tones), and the rest for transmission of service information (Pilot Tones).

In terms of modulation technology, the 802.11a protocol is not much different from 802.11g. At low bit rates, binary and quadrature phase modulation BPSK and QPSK are used to modulate subcarrier frequencies. When using BPSK modulation, only one information bit is encoded in one symbol. Accordingly, when using QPSK modulation, that is, when the signal phase can take four different values, two information bits are encoded in one symbol. BPSK modulation is used to transmit data at 6 and 9 Mbps, and QPSK modulation is used at 12 and 18 Mbps.

To transmit at higher speeds, the IEEE 802.11a standard uses 16-QAM and 64-QAM quadrature amplitude modulation. In the first case there are 16 different signal states, which allows you to encode 4 bits in one symbol, and in the second there are already 64 possible signal states, which allows you to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at 24 and 36 Mbps, and 64-QAM modulation is used at 48 and 54 Mbps.

The information capacity of an OFDM symbol is determined by the type of modulation and the number of subcarriers. Since 48 subcarriers are used for data transmission, the capacity of an OFDM symbol is 48 x Nb, where Nb is the binary logarithm of the number of modulation positions, or, more simply, the number of bits that are encoded in one symbol in one subchannel. Accordingly, the OFDM symbol capacity ranges from 48 to 288 bits.

Since the convolutional coding rate can be different, when using the same type of modulation, the data transmission rate is different.

Consider, for example, BPSK modulation, where the data rate is 6 or 9 Mbit/s. The duration of one symbol together with the guard interval is 4 μs, which means that the pulse repetition rate will be 250 kHz. Considering that one bit is encoded in each subchannel, and there are 48 such subchannels in total, we obtain that the total data transfer rate will be 250 kHz x 48 channels = 12 MHz. If the convolutional coding speed is 1/2 (one service bit is added for each information bit), the information speed will be half the full speed, that is, 6 Mbit/s. At a convolutional coding rate of 3/4, for every three information bits one service bit is added, so in this case the useful (information) speed is 3/4 of the full speed, that is, 9 Mbit/s.

Similarly, each modulation type corresponds to two different transmission rates (Table 1).

Table 1. Relationship between transmission rates
and modulation type in the 802.11a standard

Transfer rate, Mbit/s	Modulation type	Convolutional coding rate	Number of bits in one character in one subchannel	Total number of bits in a symbol (48 subchannels)	Number of information bits in a symbol

After convolutional encoding, the bit stream is subjected to interleaving, or interleaving. Its essence is to change the order of bits within one OFDM symbol. To do this, the sequence of input bits is divided into blocks whose length is equal to the number of bits in the OFDM symbol (NCBPS). Next, according to a certain algorithm, a two-stage rearrangement of bits in each block is performed. In the first stage, the bits are rearranged so that adjacent bits are transmitted on non-adjacent subcarriers when transmitting an OFDM symbol. The bit swapping algorithm at this stage is equivalent to the following procedure. Initially, a block of bits of length NCBPS is written row by row into a matrix containing 16 rows and NCBPS/16 rows. Next, the bits are read from this matrix, but in rows (or in the same way as they were written, but from a transposed matrix). As a result of this operation, initially adjacent bits will be transmitted on non-adjacent subcarriers.

This is followed by a second bit permutation step, the purpose of which is to ensure that adjacent bits do not simultaneously appear in the least significant bits of the groups defining the modulation symbol in the signal constellation. That is, after the second stage of permutation, adjacent bits appear alternately in the high and low digits of the groups. This is done in order to improve the noise immunity of the transmitted signal.

After interleaving, the bit sequence is divided into groups according to the number of positions of the selected modulation type and OFDM symbols are formed.

The generated OFDM symbols are subjected to fast Fourier transform, resulting in the formation of output in-phase and quadrature signals, which are then subjected to standard processing - modulation.

IEEE 802.11n standard

Development of the IEEE 802.11n standard officially began on September 11, 2002, that is, one year before the final adoption of the IEEE 802.11g standard. In the second half of 2003, the IEEE 802.11n Task Group (802.11 TGn) was created, whose task was to develop a new wireless communication standard at speeds above 100 Mbit/s. Another task group, 802.15.3a, also dealt with the same task. By 2005, the processes of developing a single solution in each of the groups had reached a dead end. In the 802.15.3a group, there was a confrontation between Motorola and all other members of the group, and members of the IEEE 802.11n group split into two approximately identical camps: WWiSE (World Wide Spectrum Efficiency) and TGn Sync. The WWiSE group was led by Aigro Networks, and the TGn Sync group was led by Intel. In each of the groups, for a long time, none of alternative options could not get the 75% of votes necessary for his approval.

After almost three years of unsuccessful opposition and attempts to work out a compromise solution that would suit everyone, the 802.15.3a group members voted almost unanimously to eliminate the 802.15.3a project. Members of the IEEE 802.11n project turned out to be more flexible - they managed to agree and create a unified proposal that would suit everyone. As a result, on January 19, 2006, at a regular conference held in Kona, Hawaii, a draft specification of the IEEE 802.11n standard was approved. Of 188 members working group 184 were in favor of adopting the standard, and four abstained. The main provisions of the approved document will form the basis for the final specification of the new standard.

The IEEE 802.11n standard is based on OFDM-MIMO technology. Many of the technical details implemented in it are borrowed from the 802.11a standard, but the IEEE 802.11n standard provides for the use of both the frequency range adopted for the IEEE 802.11a standard and the frequency range adopted for the IEEE 802.11b/g standards. That is, devices that support the IEEE 802.11n standard can operate in either the 5 or 2.4 GHz frequency range, with the specific implementation depending on the country. For Russia, IEEE 802.11n devices will support the 2.4 GHz frequency range.

The increase in transmission speed in the IEEE 802.11n standard is achieved, firstly, by doubling the channel width from 20 to 40 MHz, and secondly, by implementing MIMO technology.

MIMO (Multiple Input Multiple Output) technology involves the use of multiple transmitting and receiving antennas. By analogy, traditional systems, that is, systems with one transmitting and one receiving antenna, are called SISO (Single Input Single Output).

Theoretically, a MIMO system with n transmitting and n receiving antennas can provide peak throughput in n times larger than SISO systems. This is achieved by the transmitter breaking the data stream into independent bit sequences and transmitting them simultaneously using an array of antennas. This transmission technique is called spatial multiplexing. Note that all antennas transmit data independently of each other in the same frequency range.

Consider, for example, a MIMO system consisting of n transmitting and m receiving antennas (Fig. 2).

Rice. 2. Implementation principle of MIMO technology

The transmitter in such a system sends n independent signals using n antennas On the receiving side, each m antenna receives signals that are a superposition n signals from all transmitting antennas. So the signal R1, received by the first antenna, can be represented as:

Writing similar equations for each receiving antenna, we obtain the following system:

Or, rewriting this expression in matrix form:

Where [ H] - transfer matrix describing the MIMO communication channel.

In order for the decoder on the receiving side to be able to correctly reconstruct all signals, it must first determine the coefficients hij, characterizing each of m x n transmission channels. To determine the coefficients hij MIMO technology uses a packet preamble.

Having determined the coefficients of the transfer matrix, you can easily restore the transmitted signal:

Where [ H]–1 - matrix inverse to the transfer matrix [ H].

It is important to note that in MIMO technology, the use of multiple transmitting and receiving antennas makes it possible to increase the throughput of a communication channel by implementing several spatially separated subchannels, while data is transmitted in the same frequency range.

MIMO technology does not affect the data encoding method in any way and, in principle, can be used in combination with any methods of physical and logical data encoding.

MIMO technology was first described in the IEEE 802.16 standard. This standard allows the use of MISO technology, that is, several transmitting antennas and one receiving antenna. The IEEE 802.11n standard allows up to four antennas at the access point and wireless adapter. Mandatory mode implies support for two antennas at the access point and one antenna and wireless adapter.

The IEEE 802.11n standard provides both standard 20 MHz and double-width channels. However, the use of 40 MHz channels is an optional feature of the standard, since the use of such channels may contravene the laws of some countries.

The 802.11n standard provides two transmission modes: standard transmission mode (L) and high throughput (HT) mode. In traditional transmission modes, 52 frequency OFDM subchannels (frequency subcarriers) are used, of which 48 are used for data transmission, and the rest for transmission of service information.

In modes with increased throughput with a channel width of 20 MHz, 56 frequency subchannels are used, of which 52 are used for data transmission, and four channels are pilot. Thus, even when using a 20 MHz channel, increasing the frequency subchannels from 48 to 52 increases the transmission speed by 8%.

When using a double-width channel, that is, a 40 MHz channel, in standard transmission mode the broadcast is actually carried out on a double channel. Accordingly, the number of frequency subcarriers doubles (104 subchannels, of which 96 are information). Thanks to this, the transfer speed increases by 100%.

When using a 40-MHz channel and high-bandwidth mode, 114 frequency subchannels are used, of which 108 are information subchannels and six are pilot ones. Accordingly, this allows you to increase the transmission speed by 125%.

Table 2. Relationship between transmission rates and modulation type
and convolutional coding speed in the 802.11n standard
(20 MHz channel width, HT mode (52 frequency subchannels))

Modulation type	Convolutional coding rate	Number of bits in one symbol in one subchannel	Total number of bits in an OFDM symbol	Number of information bits per symbol	Data transfer rate
Modulation type	Convolutional coding rate	Number of bits in one symbol in one subchannel	Total number of bits in an OFDM symbol	Number of information bits per symbol

Two more circumstances due to which the IEEE 802.11n standard increases the transmission speed are a reduction in the duration of the GI guard interval in OGDM symbols from 0.8 to 0.4 μs and an increase in the speed of convolutional coding. Recall that in the IEEE 802.11a protocol, the maximum convolutional coding rate is 3/4, that is, for every three input bits one more is added. In the IEEE 802.11n protocol, the maximum convolutional coding rate is 5/6, that is, every five input bits in the convolutional encoder are converted into six output bits. The relationship between transmission rates, modulation type and convolutional coding rate for a standard 20 MHz wide channel is given in Table. 2.

The popularity of Wi-Fi connections is growing every day, as the demand for this type of network is increasing at a tremendous pace. Smartphones, tablets, laptops, monoblocks, TVs, computers - all our equipment supports a wireless Internet connection, without which it is no longer possible to imagine the life of a modern person.

Data transmission technologies are developing along with the release of new equipment

In order to choose the right network for your needs, you need to find out about everything Wi-Fi standards existing today. The Wi-Fi Alliance has developed more than twenty connection technologies, four of which are most in demand today: 802.11b, 802.11a, 802.11g and 802.11n. The manufacturer's most recent discovery was the 802.11ac modification, the performance of which is several times higher than the characteristics of modern adapters.

Is a senior certified technology wireless connection and is characterized by general accessibility. The device has very modest parameters:

Information transfer speed - 11 Mbit/s;
Frequency range - 2.4 GHz;
The range of action (in the absence of volumetric partitions) is up to 50 meters.

It should be noted that this standard has poor noise immunity and low throughput. Therefore, despite the attractive price of this Wi-Fi connection, its technical component lags significantly behind more modern models.

802.11a standard

This technology is an improved version of the previous standard. The developers focused on the device’s throughput and clock speed. Thanks to such changes, this modification eliminates the influence of other devices on the quality of the network signal.

Frequency range - 5 GHz;
The range is up to 30 meters.

However, all the advantages of the 802.11a standard are compensated equally by its disadvantages: a reduced connection radius and a high (compared to 802.11b) price.

802.11g standard

The updated modification becomes a leader in today's wireless network standards, since it supports work with the widespread 802.11b technology and, unlike it, has a fairly high connection speed.

Information transfer speed - 54 Mbit/s;
Frequency range - 2.4 GHz;
Range of action - up to 50 meters.

As you may have noticed, the clock frequency has dropped to 2.4 GHz, but the network coverage has returned to its previous levels typical for 802.11b. In addition, the price of the adapter has become more affordable, which is a significant advantage when choosing equipment.

802.11n standard

Despite the fact that this modification has been on the market for a long time and has impressive parameters, manufacturers are still working on improving it. Due to the fact that it is incompatible with previous standards, its popularity is low.

Information transfer speed is theoretically up to 480 Mbit/s, but in practice it is half as much;
Frequency range - 2.4 or 5 GHz;
Range of action - up to 100 meters.

Since this standard is still evolving, it has its own characteristics: it may conflict with equipment that supports 802.11n only because the device manufacturers are different.

Other standards

In addition to popular technologies, Wi-Fi manufacturer The Alliance has developed other standards for more specialized applications. Such modifications that perform service functions include:

802.11d- does compatible devices wireless communications from different manufacturers, adapts them to the peculiarities of data transmission at the countrywide level;
802.11e- determines the quality of sent media files;
802.11f- manages a variety of access points from different manufacturers, allows you to work equally in different networks;

802.11h- prevents loss of signal quality due to the influence of meteorological equipment and military radars;
802.11i- improved version of protecting users’ personal information;
802.11k- monitors the load on a specific network and redistributes users to other access points;
802.11m- contains all corrections to 802.11 standards;
802.11p- determines the nature of Wi-Fi devices located within a range of 1 km and moving at speeds of up to 200 km/h;
802.11r- automatically finds wireless network in roaming and connects mobile devices to it;
802.11s- organizes a full mesh connection, where each smartphone or tablet can be a router or connection point;
802.11t- this network tests the entire 802.11 standard, provides testing methods and their results, and sets requirements for the operation of the equipment;
802.11u- this modification is known to everyone from the development of Hotspot 2.0. It ensures the interaction of wireless and external networks;
802.11v- this technology creates solutions to improve 802.11 modifications;
802.11y- unfinished technology linking frequencies 3.65–3.70 GHz;
802.11w- the standard finds ways to strengthen the protection of access to information transmission.

The latest and most technologically advanced standard 802.11ac

802.11ac modification devices provide users with a completely new quality of Internet experience. Among the advantages of this standard, the following should be highlighted:

High speed. When transmitting data over the 802.11ac network, wider channels and higher frequencies are used, which increases the theoretical speed to 1.3 Gbps. In practice, throughput is up to 600 Mbit/s. In addition, an 802.11ac-based device transmits more data per clock cycle.

Increased number of frequencies. The 802.11ac modification is equipped with a whole range of 5 GHz frequencies. The latest technology has a stronger signal. The high range adapter covers a frequency band up to 380 MHz.
802.11ac network coverage area. This standard provides a wider network range. In addition, the Wi-Fi connection works even through concrete and plasterboard walls. Interference that occurs during the operation of home appliances and the neighbor’s Internet does not in any way affect the operation of your connection.
Updated technologies. 802.11ac is equipped with the MU-MIMO extension, which ensures smooth operation of multiple devices on the network. Beamforming technology identifies the client's device and sends several streams of information to it at once.

Having become more familiar with all the Wi-Fi connection modifications that exist today, you can easily choose the network that suits your needs. Please remember that most devices contain a standard 802.11b adapter, which is also supported by 802.11g technology. If you are looking for an 802.11ac wireless network, the number of devices equipped with it today is small. However, this is very current problem and soon all modern equipment will switch to the 802.11ac standard. Do not forget to take care of the security of Internet access by installing complex code to your Wi-Fi connection and antivirus to protect your computer from virus software.

Currently, mainly three standards of the IEEE 802.11 group are widely used (presented in Table 1.1)

Table 1.1 - Main characteristics of IEEE 802.11 group standards

Standard
Frequency range, GHz			2.4 or 5.0
Transfer Method
Speed, Mbit/s
Compatibility
Modulation method	BPSK, QPSK OFDM	BPSK, QPSK OFDM
Communication range indoors, m
Outdoor communication range, m

1.3.1 IEEE 802.11g standard

The IEEE 802.11g standard, adopted in 2003, is a logical development of the 802.11b standard and involves data transmission in the same frequency range, but at higher speeds. Additionally, 802.11g is fully compatible with 802.11b, meaning any 802.11g device must be able to work with 802.11b devices. The maximum data transfer rate in the 802.11g standard is 54 Mbit/s. During the development of the 802.11g standard, two competing technologies were considered: the OFDM orthogonal frequency division method, borrowed from the 802.11a standard and proposed for consideration by Intersil, and the PBCC binary packet convolutional coding method, proposed by Texas Instruments. As a result, the 802.11g standard contains a compromise solution: OFDM and CCK technologies are used as base technologies, and the optional use of PBCC technology is provided.

The idea of convolutional coding (Packet Binary Convolutional Coding, PBCC) is as follows. The incoming sequence of information bits is converted in a convolutional encoder so that each input bit corresponds to more than one output bit. That is, the convolutional encoder adds certain redundant information to the original sequence. If, for example, each input bit corresponds to two output bits, then we talk about convolutional coding with a rate of 1/2. If every two input bits correspond to three output bits, then the convolutional coding speed will be 2/3.

Any convolutional encoder is built on the basis of several sequentially connected memory cells and XOR gates. The number of storage cells determines the number of possible encoder states. If, for example, a convolutional encoder uses six memory cells, then the encoder stores information about six previous signal states, and taking into account the value of the input bit, we obtain that such an encoder uses seven bits of the input sequence. Such a convolutional encoder is called a seven-state encoder.

Optionally, in the 802.11g protocol, PBCC technology can be used at transmission rates of 22 and 33 Mbit/s.

PBCC technology is optional in the IEEE 802.11g standard, and OFDM technology is mandatory. In order to understand the essence of OFDM technology, let's take a closer look at the multipath interference that occurs when signals propagate in an open environment.

For this reason, at high data rates, a data encoding method called Orthogonal Frequency Division Multiplexing (OFDM) is used. Its essence lies in the fact that the stream of transmitted data is distributed over many frequency subchannels and transmission is carried out in parallel on all such subchannels. In this case, a high transmission speed is achieved precisely due to the simultaneous transmission of data over all channels, while the transmission speed in a separate subchannel may be low.

Due to the fact that the data transmission rate in each of the frequency subchannels can be made not too high, the prerequisites are created for effective suppression of intersymbol interference.

This method of dividing a wideband channel into orthogonal frequency subchannels is called orthogonal frequency division multiplexing (OFDM). To implement it in transmitting devices, an inverse fast Fourier transform (IFFT) is used, which transforms the previously multiplexed n -channels signal from time O th representation into frequency.

The guard interval creates pauses between individual symbols, and if its duration exceeds the maximum signal delay time due to multipath propagation, then intersymbol interference does not occur.

1.3.2 IEEE 802.11a standard

The IEEE 802.11a standard provides data transfer rates of up to 54 Mbit/s. Unlike the basic standard, the 802.11a specifications provide for operation in the new 5 GHz frequency range. Orthogonal frequency multiplexing (OFDM) was chosen as a signal modulation method, which ensures high communication stability in conditions of multipath signal propagation.

In accordance with FCC rules, the UNII frequency range is divided into three 100-MHz sub-bands, differing in maximum emission power limits. The low band (5.15 to 5.25 GHz) provides only 50 mW of power, the middle (5.25 to 5.35 GHz) 250 mW, and the high (5.725 to 5.825 GHz) 1 W. The use of three frequency subbands with a total width of 300 MHz makes the IEEE 802.11a standard the most broadband of the 802.11 family of standards and allows the entire frequency range to be divided into 12 channels, each of which has a width of 20 MHz, with eight of them lying in the 200 MHz range from 5 .15 to 5.35 GHz, and the remaining four channels are in the 100 MHz range from 5.725 to 5.825 GHz (Figure 1.3). At the same time, the four upper frequency channels, which provide the highest transmission power, are used primarily for transmitting signals outdoors.

Figure 1.3 - Division of the UNII range into 12 frequency subbands

The sequence of processing input data (bits) in the IEEE 802.11a standard is as follows. Initially, the input data stream is subjected to a standard scrambling operation. After this, the data stream is fed to the convolutional encoder. The convolutional coding rate (in combination with puncture coding) can be 1/2, 2/3 or 3/4. Since the convolutional coding rate can be different, when using the same type of modulation, the data transmission rate is different. Consider, for example, BPSK modulation, where the data rate is 6 or 9 Mbit/s. The duration of one symbol together with the guard interval is 4 μs, which means that the pulse repetition rate will be 250 kHz. Considering that one bit is encoded in each subchannel, and there are 48 such subchannels in total, we obtain that the total data transfer rate will be 250 kHz x 48 channels = 12 MHz. If the convolutional coding speed is 1/2 (one service bit is added for each information bit), the information speed will be half the full speed, that is, 6 Mbit/s. At a convolutional coding rate of 3/4, for every three information bits one service bit is added, so in this case the useful (information) speed is 3/4 of the full speed, that is, 9 Mbit/s. Similarly, each modulation type corresponds to two different transmission rates (Table 1.2).

Table 1.2 - Relationship between transmission rates and modulation type in the 802.11a standard

Transfer rate, Mbit/s	Modulation type	Convolutional coding rate	Number of battles per symbol in one subchannel	Total number of bits per symbol (48 subchannel)	Number of information bits in a symbol

After interleaving, the bit sequence is divided into groups according to the number of positions of the selected modulation type and OFDM symbols are formed.

The generated OFDM symbols are subjected to fast Fourier transform, resulting in the formation of output in-phase and quadrature signals, which are then subjected to standard processing - modulation.

1.3.3 IEEE 802.11n standard

This standard was approved on September 11, 2009. 802.11n is comparable in transmission speed to wired standards. The maximum transfer speed of the 802.11n standard is approximately 5 times higher than the performance of classic Wi-Fi.

The following main advantages of the 802.11n standard can be noted:

– high data transfer speed (about 300 Mbit/s);

– uniform, stable, reliable and high-quality coverage of the station’s coverage area, the absence of uncovered areas;

– compatibility with previous versions Wi-Fi standard.

Flaws:

– high power consumption;

– two operating ranges (possible replacement of equipment);

– more complicated and larger equipment.

The increase in transmission speed in the IEEE 802.11n standard is achieved, firstly, by doubling the channel width from 20 to 40 MHz, and secondly, by implementing MIMO technology.

Figure 1.4 - Implementation principle of MIMO technology

The transmitted sequence is divided into parallel streams, from which the original signal is restored at the receiving end. This is where some complexity arises - each antenna receives a superposition of signals that must be separated from each other. For this purpose, a specially developed spatial signal detection algorithm is used at the receiving end. This algorithm is based on the allocation of subcarriers and turns out to be more complex, the greater their number. The only disadvantage of using MIMO is the complexity and bulk of the system and, as a result, higher energy consumption. To ensure compatibility between MIMO stations and traditional stations, three operating modes are provided:

Legacy mode.

Mixed mode.

Green field mode.

Each operating mode has its own preamble structure - a service field of the packet that indicates the start of transmission and serves to synchronize the receiver and transmitter. The preamble contains information about the packet length and its type, including the type of modulation, the selected encoding method, and all encoding parameters. To avoid conflicts in the operation of MIMO and conventional stations (with one antenna), during exchange between MIMO stations, the packet is accompanied by a special preamble and header. Upon receiving such information, stations operating in legacy mode defer transmission until the end of the session between MIMO stations. In addition, the preamble structure defines some of the primary tasks of the receiver, such as estimating received signal strength for automatic gain control, detecting the start of a packet, and time and frequency offset.

Operating modes of MIMO stations.

Legacy mode. This mode is designed to ensure exchange between two stations with one antenna. Information is transmitted via 802.11a protocols. If the transmitter is a MIMO station and the receiver is a regular station, then the transmitting system uses only one antenna and the transmission process is the same as in previous versions of the Wi-Fi standard. If the transmission goes in the opposite direction - from a conventional station to a multi-antenna station, then the MIMO station uses many receiving antennas, but in this case the transmission speed is not maximum. The preamble structure in this mode is the same as in the 802.11a version.

Mixed mode. In this mode, exchange is carried out both between MIMO systems and between conventional stations. Because of this, MIMO systems generate two types of packets, depending on the type of receiver. Conventional stations are slow because they don't support high speeds, while inter-MIMO is much faster, but the transfer speed is lower than greenfield mode. The preamble in the packet from a regular station is the same as in the 802.11a standard, but in the MIMO packet it is slightly modified. If the transmitter is a MIMO system, then each antenna does not transmit the entire preamble, but a cyclically shifted one. Due to this, the power consumption of the station is reduced, and the channel is used more efficiently. However, not all legacy stations can operate in this mode. The fact is that if the device synchronization algorithm is based on cross-correlation, then loss of synchronization will occur.

Green field mode. This mode takes full advantage of MIMO systems. Transmission is only possible between multi-antenna stations with legacy receivers. When a MIMO system is transmitting, conventional stations wait until the channel is free to avoid collisions. In green field mode, signal reception from systems operating according to the first two schemes is possible, but transmission to them is not. This is done in order to exclude single-antenna stations from the exchange and thereby increase the speed of operation. The packets are accompanied by preambles, which are only supported by MIMO stations. All these measures allow you to get the most out of MIMO-OFDM systems. All modes of operation must be protected from interference from adjacent station operation to prevent signal distortion. At the physical layer of the OSI model, special fields are used for this in the preamble structure, which notify the station that a transmission is in progress and a certain waiting time is required. Some protection methods are also adopted at the data link layer. Depending on the bandwidth used, operating modes are classified as follows:

1. Inherited mode. This mode is needed to harmonize with previous versions of Wi-Fi. It is very similar to 802.11a/g both in hardware and bandwidth, which is 20 MHz.

2. Double inherited mode. The devices use a 40 MHz bandwidth, with the same data sent on the upper and lower channels (each 20 MHz wide), but with a 90° phase shift. The structure of the packet is based on the fact that the receiver is a regular station. Signal duplication reduces distortion, thereby increasing transmission speed.

3. High throughput mode. The devices support both frequency bands - 20 and 40 MHz. In this mode, stations exchange only MIMO packets. Network speed is maximum.

4. Top channel mode. This mode uses only the upper half of the 40 MHz band. Stations can exchange any packets.

5. Bottom channel mode. This mode uses only the lower half of the 40 MHz band. Stations can also exchange any packets.

Methods for increasing performance.

The data transfer speed depends on many factors (Table 1.3) and, above all, on the bandwidth. The wider it is, the higher the exchange speed. The second factor is the number of parallel threads. In the 802.11n standard, the maximum number of channels is 4. The type of modulation and coding method are also of great importance. Anti-jamming codes that are typically used in networks require some redundancy. If there are too many security bits, the transmission speed of useful information will decrease. In the 802.11n standard, the maximum relative encoding rate is up to 5/6, that is, there is one redundant bit for every 5 data bits. Table 3 shows the exchange rates for QAM and BPSK quadrature modulation. It can be seen that with other identical parameters, QAM modulation provides much higher operating speed.

Table 1.3 - Data transfer rate for different types of modulation

802.11n transmitters and receivers

The IEEE 802.11n standard allows the use of up to four antennas at the access point and wireless adapter. Mandatory mode implies support for two antennas at the access point and one antenna and wireless adapter. The IEEE 802.11n standard provides both standard 20 MHz and double-width channels. The general block diagram of the transmitter is shown in Figure 1.5. The transmitted data passes through a scrambler, which inserts extra zeros or ones into the code (called pseudo-random noise masking) to avoid long sequences of identical characters. The data is then divided into N streams and sent to a forward error correction (FEC) encoder. For systems with one or two antennas, N = 1, and if three or four transmit channels are used, then N = 2.

Figure 1.5 - General structure of a MIMO-OFDM transmitter

The encoded sequence is divided into separate spatial streams. The bits in each stream are interleaved (to eliminate block errors) and then modulated. Next, space-time streams are formed, which pass through the inverse fast Fourier transform block and arrive at the antennas. The number of space-time streams is equal to the number of antennas. The structure of the receiver is similar to the structure of the transmitter shown in Figure 1.6, but all actions are performed in the reverse order.

Figure 1.6 - General structure of a MIMO-OFD receiver